As a consequence of the progress in the development of semiconductor components, a constantly increasing trend towards direct-current arc furnaces for melting and heating steel has ensued. Typically, the arc is formed between a cathode, which is disposed above the melt and protrudes into the vessel, and the melt itself. The direct current is returned by one or more anodes disposed in the region of the bottom of the vessel.
Since these anodes are subjected to a continuing high thermal stress, graphite is regularly used as anode material. These graphite electrodes characteristically have a high softening point and a high melting point. However, they possess the disadvantage that the graphite dissolves in contact with steel of low carbon content. This leads not only to high wear of the anode, which at the same time functions as a protective lining, but also to an impermissible carburization of the steel melt. The same is true for anodes of graphite-magnetite compositions, although it has been possible to reduce the carburization of the steel melt with such anodes.
Metallic electrodes are also used in direct-current arc furnaces. However, metallic electrodes are excluded from being used in ladle furnaces, since the metallic electrodes must be cooled intensively and a melting of the electrodes up to the melt break-out is to be avoided if for no other reason than work safety. Cooling of such metallic electrodes must be kept up during the entire period in which the melt is in the ladle. This leads to a high cost for keeping up the supply of cooling medium during the transport, possibly over long distances, from the smelting furnace to the converter or the ladle furnace, or it leads to structurally expensive electrodes such as shown in EP 0 167 037 B1.
The specification of EP 00 124 490 A1 discloses a direct-current anode comprising a prefabricated, removable, electrically conductive element which is in contact with metal bath. The element in this case is connected to an electrically conductive, grainy material, which lies on a metal plate having a metal rod. The prefabricated element can also consist of a refractory cast part with interspersed metal rods. The anode taught by this publication is different in structure and composition from the rest of the bottom of the direct-current arc furnace. Moreover, the metal rods embedded in the refractory casting have direct contact with the melt.
The German Offenlegungsschrift 27 53 198 discloses an electrode of a generator for the magneto-hydrodynamic generation of power. This electrode has a refractory ceramic matrix which contains 10 to 15% by volume of a continuous metal reinforcement. The surface of this electrode can consist of a refractory ceramic matrix which has not been reinforced. The place of use is a plasma, which may be an ionized or inert gas, in which temperatures up to 2,800.degree. C. may be encountered. This leads to electrode-plasma temperatures of up to 2,100.degree. C. Gas flows around this electrode which is standing apart and is in an environment which cannot be compared with a smelting vessel for metallic materials.